Caesar cipher +5

[blue10]

I am completely new to the programming world and I am having trouble writing this code correctly. I am supposed to shift the letters of the alphabet 5 spaces to the right. I have come up with two equations. if c<(u) ascii(117) then I simply add 5. but if c>= v(ascii 118) then I subtract 21. I am trying to write this in if statements. thank you in advance!

#include <iostream>
using namespace std ;
main()
{
 int c;
 int shift= 5;
 {
     if (c + 5 > 122 )
        c = (c+ shift);
     else if(c == 32)
        c = c;
     else if (c < 117)
        c= (c + 5);
     else if c >= 118)
        c= (c- 21);
     else if (c== '\0')
              break;
     else
        c= c + shift;
 }
  

[Catfish666]

The ASCII table is available here if you need it:
http://www.cplusplus.com/doc/ascii/

That said, if you want to implement the Caesar cipher a more elegant approach is to not rely on ASCII codes at all. Instead do what would be done in real life: use an alphabet and shift it.

OA: Original Alphabet
SA: Shifted Alphabet (in this example, shifted by 5)

OA: abcdefghijklmnopqrstuvwxyz
SA: fghijklmnopqrstuvwxyzabcde

After we construct the Shifted Alphabet, all we have to do is:
1) go through the Message one letter at a time
2) find the position P of the letter in the Original Alphabet
3) replace the letter in the Message with the letter at position P in the Shifted Alphabet

OM: Original Message
EM: Encrypted Message

    0         1         2
    01234567890123456789012345 (counting from 0 to 25)
OA: abcdefghijklmnopqrstuvwxyz
SA: fghijklmnopqrstuvwxyzabcde

OM: hello
EM: ?????

read letter in OM: h
in OA, letter h is at position 7
in SA, at position 7 is letter m
write letter in EM: m

and so on, until we complete EM:

EM: mjqqt

Creating the Original Alphabet and Shifted Alphabet are both very easy to do:

#include <algorithm>
#include <string>

// ...

std::string OA("abcdefghijklmnopqrstuvwxyz");
std::string SA(OA); // first copy the OA

std::rotate(SA.begin(), SA.begin() + 5, SA.end());

http://www.cplusplus.com/reference/algorithm/rotate/

Here's an online tool you can use to test that your algorithm is correct:
http://www.simonsingh.net/The_Black_Chamber/caesar.html

[blue10]

Thank you very much!
I did it both ways and I got the concept and the practice down!

[Catfish666]

blue10 wrote:

I did it both ways and I got the concept and the practice down!

If you would like help with refining your programs, please post your code for others to review.

[blue10]

Here is my finished code for this project

#include <iostream>
using namespace std ;
main()
{
 int myinput;// enter value
 int shift= 13; // shift alphabet 13 to the right
 myinput = cin.get();
 while (! cin.eof())
 {

    if (myinput == 32)
        myinput = myinput ;// input spaces, output spaces
    else if (myinput >=97 and myinput<110)//letter is between 'a' and 'n'
        myinput+=shift;// if letter is less than 'n' shift 13 spaces
    else if (myinput<= 122 and myinput >= 110)//letter is between 'n' and 'z'
         myinput-= shift ;// if letter is greater than or equal to 'n', subtract 13 spaces

    else if (myinput>= 65 and myinput<78)//letter is between 'A' and 'N'
        myinput+=shift;//if letter is less than 'N' shift 13 spaces
    else if (myinput<=90 and myinput >= 78)//letter is between 'N' and 'Z'
         myinput-=shift ;//if letter is greater than or equal to 'N', subtract 13 spaces

    cout.put(myinput);// put 'myinput' into program
    myinput= cin.get();
 }
 myinput=cin.get();
}

[giblit]

Another tip you can make it more readable if you change your if statements from the ascii code to the actual characters.

    if (myinput == ' ')
        myinput = myinput ;// input spaces, output spaces
    else if (myinput >= 'a' && myinput< 'n')//letter is between 'a' and 'n'
        myinput+=shift;// if letter is less than 'n' shift 13 spaces
    else if (myinput<= 'n' && myinput >= 'z')//letter is between 'n' and 'z'
         myinput-= shift ;// if letter is greater than or equal to 'n', subtract 13 spaces

    else if (myinput>='A' && myinput< 'N')//letter is between 'A' and 'N'
        myinput+=shift;//if letter is less than 'N' shift 13 spaces
    else if (myinput<= 'N' && myinput >= 'Z')//letter is between 'N' and 'Z'
         myinput-=shift ;//if letter is greater than or equal to 'N', subtract 13 spaces 

Another thing to mention is you can do this with 3 if's instead of 5 if you combine the a-n with A-N and the n-z with N-Z. Since they both do the same thing.